1. Field of the Invention
This invention generally relates to data processing systems that have the capability of making independent data copies and more particularly to a method and apparatus for updating those copies on a differential basis.
2. Description of Related Art
U.S. Pat. No. 6,363,385, to Kedem et al. discloses a method and apparatus for copying portions of a data set from one logical volume to another in one or more data storage facilities. More specifically, in accordance with the described method and apparatus, portions of a data file residing in a defined extent of tracks are copied from a source storage device to a destination storage device in response to a copy command from a requesting host application. The command identifies the source file and the storage locations in a destination storage device. An extents track is formed to establish an environment in which the file will be copied. The calling system receives an immediate response to the command that the copy operation is complete even though no data has been copied.
Application programs may access the file in either the source storage device or the destination storage device at any time during the process. A copy program transfers the file portions on a track-by-track basis to the destination storage device. Procedures assure that any data access to a particular track in either the source or destination storage device by any application prior to the transfer of that track is accommodated to maintain data integrity. In Symmetrix data storage facilities available from the assignee of this invention, this copy operation is called a “SNAP” operation and is requested by issuing a “FILE SMMF” command.
Sometimes it is desired to make successive copies at different times or after successive intervals. With the foregoing system, it is necessary to initiate the same FILE SMMF command for each copy. Consequently each successive copying operation copies all the data in the defined extent even though changes may only be limited to a few of many tracks. As will also be apparent, each copying requires resources from the data storage facility. As such data storage facilities become more sophisticated, excessive and unnecessary use of those resources becomes a detriment. So it would be advantageous to provide a differential copying operation for use during second and subsequent copying operations that transfers only changed data.
One obvious approach for minimizing resource utilization might include using information in the track change tables associated with the FILE SMMF command to generate new extents at the time of a second and subsequent copying operation. However, it is possible to have a large number of extents. If every other track were changed over an interval of time, for example, a 32,000-track system would require 16,000 extents. As disclosed in U.S. Pat. No. 6,363,385, the process of making a copy incorporates the step of blocking access to each extent track during initial processing. That block, or lock, requires time and could introduce unacceptable delays whenever the number of extent tracks reached a large number. Such an approach would also be difficult to implement in existing systems.